I knew there was some kind of internal torment for baby Schala, I tried not to think about it but you went and confirmed my worst fear. I would say that Magus could just open a time portal to just before he handed his sister over to Ridley and tell himself not to do that, but I'm not sure how the rules of time travel work in Chrono Trigger.

They work like this, you can meet your past selves, as evidenced by when Magus wound up back in Zeal with his younger self present. He could warn himself, but I'm not sure he actually opened a time portal before, if he could he probably would have.

Actualy, in CC, and I think sounds good for time travle in general, the rules are that you can not travel to any SPECIFIC time, but instead, to a abstract time era. Meaning, if you wanted to transport somewhere, you'll only be able to get as close to that era as the 'Entity' lets you, anywhere between a week, to 10,000,000 years.